Crack Paths 2006

1200

10123456890700 1

10

100

1000

10000

P a )

(M

rest e s s

457273K

F r a c t u

673K

723K

973K

102

103

104

Timeto fracture (min)

Fig.2 Delayed fracture curve of hydrogen charged specimen (Bolten110N).

To have an extensive understanding on the nature of delayed fracture, the threshold

stress (th) was normalized by the yield strength (y) as shown in Fig.3.

01.012345678900 0 200 400 600 800 1000 1200 1400 S35C

Bolten110N

a c t u r e (

delaye f r

c e o f

R e s i s t a n

)

Yield strength (MPa)

Fig.3 Relationship between resistance of delayed fracture and yield strength.

The results show that the normalized delayed fracture strength decreases with the

increase in the yield strength, and a remarkable change in fracture susceptibility can be

seen at about the yield strength level of 1000MPa. Then, to examine the detailed aspects

of crack development during the fracture process in all range of yield strength,

fractographic analysis was made with SEM.

Difference in Morphology of Crack Development in High and Low Strength Steels

It has long been considered that the characteristic feature of crack growth under